Newton Cotes for mass matrix

I am using Newton Cotes and Gauss integration rule for computing velocity mass matrix. I appear that mass matrix computed with Newton Cotes give some negative entries on the main diagonal while Gauss does not. In my case, mass matrix computed with Newton Cotes works better.

I am using Newton Cotes and Gauss integration rule for computing velocity mass matrix in 3D for solving steady inomp. NS problem. It appear that mass matrix computed with Newton Cotes give some negative entries on the main diagonal while Gauss does not. In my case, mass matrix computed with Newton Cotes works better.